Apparatus for forming refractory masses

ABSTRACT

Apparatus for forming refractory masses, in situ, at selected surface locations by applying to such location a mixture of fine particles of a substance which is capable of undergoing combustion to produce heat and particles of another substance, the substances being of such composition that such heat will melt at least the surfaces of the particles of the other substance, and creating the conditions for producing such combustion.

United States Patent 11 1 Brichard et al. Apr. 2, 1974 [54] APPARATUS FOR FORMING REFRACTORY 3,518,804 8/1950 Marvin 431/22 MASSES 3,627,965 12/1971 Zweig 2,964,102 12/1960 Cassell et a1 [75] In t s: dga Brichard, Ransart; Maurice 3,637,135 7 1972 Luderer 222/193 x Jaupain, Jumet; Emile Plumat, 3,598,635 8/1971 Sagona 264/30 X Gilly; Pierre Deschepper, Marcinelle, all of Belgium OTHER PUBLICATIONS [73] Assignee: Glaverbel S.A., Brussels, Belgium Metco 1nc., Type 2M Plasma Flame Spray System In- 12 Filed: J 1972 structions, N.Y., 1963, pp. 38 & 43. [21] Appl. No.: 259,991

Related US. Application Data Primary ExaminerStanley H. Tollberg [62] Division of Sci. NO. 86,532, Nov. 3, 1970, Pat. N0. Assistant Examiner-la")! Marti" 3,684,560. Attorney, Agent, or Firm-Spencer & Kaye [30] Foreign Application Priority Data Nov. 4, 1969 Luxembourg 59745 57] ABSTRACT U-S. Apparatus for forming refractory masses in sit at e. 222/193, 239/79 lected surface locations byapplying to such location a mixture of fine particle of a ubstance is capa- [58] Field of Search 222/52, 53, 54, 59, 76, ble f undergoing combustion produce heat d 222/146 C, 148, 236, 242, 193, 55; 110/22 particles of another substance, the substances being of B; 239/79, 431/22, 30; 137/90 1 such composition that such heat will melt at least the 240; 264/30; 219/75, 76 surfaces of the particles of the other substance, and

creating the conditions for producing such combus- [56] References Cited tion UNITED STATES PATENTS 3,149,759 9/1964 Manley 222 193 12 Claims, 2 Drawing Figures PATENIEDAHII 2 I974 $800883- sum 1 BF 2 APPARATUS FOR FORMING REFRACTORY MASSES CROSS REFERENCE TO RELATED APPLICATION This application is a division of application Ser. No. 86,532, filed Nov. 3, 1970 now Pat. No. 3,684,560.

BACKGROUND OF THE INVENTION cess is useful, inter alia, for repairing or strengthening furnace linings in situ.

SUMMARY OF THE INVENTION vThe present invention basically resides in a process for forming a refractory mass including projecting particles of at least one oxidizable substance which burns by combining with oxygen with accompanying evolution of heat, which particles have an average size of less than 50 microns, and particles of at least one other substance against a surface and burning the projected particles of an oxidizable substance in the presence of the projected particles of the other substance, the oxidazable and/or other substance being of such compositions that under the heat-of combustion a coherent refractory mass is formed against such surface from the oxidizable substance and the other substance.

The other substance, i.e. the particles which are used in conjunction with the fine oxidizable particles having an average size less than 50 microns, may be ofa composition which also undergoes some oxidation, but for convenience in the following description the term fine oxidizable particles" is reserved for the particles of the one substance. The particles of the other substance will similarly be referred to as the other particles.

The other substance is preferably of such a composition that the surfaces of its particles melt under the heat of combustion. In that case a very strongly coherent mass can be formed. However it is possible for the other particles to be ofa composition such that the cohesion of the final refractory mass is at least in part due to a chemical reaction and bonding between the fine oxidizable particles and the other particles.

The coherent refractory mass and the surface against which such mass is formed are preferably of such compositions that the mass adheres to the surface to form a coating. This relation is normally required in the event that the process is employed for forming a refractory coating on a substrate, e.g., a furnace lining. Such an adhesion may not be essential however when using the invention to form a'refractory mass in situ in a joint or crevice of a furnace wall where the joint space or crevice may be so shaped that it will retain the refractory filling without adhesion The invention can also be used for forming refractory masses which can be removed, e.g. lifted off the surface against which they are formed, or so as to form refractory masses which by reason of the orientation of such surface fallaway from such surface under gravity for use in some other context.

It is advantageous from the point of view of convenience for the oxidizable particles and the other particles to be mixed together and projected as a mixture against the surface on which the mass is to beformed.

This procedure produces the most satisfactory results and minimum complications in bringing the different syns s ts sa sratthssv mszssa BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partly cross-sectional, partly schematic view of one arrangement for forming refractory masses according to the invention.

FIG. 2 is a circuit diagram of one embodiment of the control circuit of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT The invention is based on experiments in which refractory metal oxides in discrete form were sprayed onto refractory surfaces together with particles of a combustible substance while the latter was undergoing combustion which was producing heat sufficient under the prevailing conditions to form a coherent refractory mass, for example as a result of softening or melting of the surfaces of the refractory oxide particles. At the same time it is possible for the refractory mass to adhere to the refractory surface to form a coating. After much experimental research it has been found that for the successful formation of refractory masses by this procedure certain conditions relating to the granulometry of the sprayed mixture are of unexpected importance. In particular, the combustible particles should have a very small average grain size.

Preferably the fine oxidizable particles are burned when substantially uniformly dispersed within the mass of other particles. In order that the process can be performed in this way, the relative sizes of the fine oxidiz able particles and the other particles should be such that the fine oxidizable particles will remain substantially uniformly dispersed throughout the total particle mixture during projection of the mixture against the surface. The required results can be achieved, for example, by employing the preferred grain sizes for the starting materials, which will be specified below.

The combustion of the fine oxidizable particles may take place in air or some other oxygen-containing mixture. Preferably however the combustion takes place in oxygen alone. In any event the use of a substantial excess of oxygen is advantageous if complete combustion of the fine oxidizable particles is required.

According to an optional but particularly advantageous feature, the combustion-supporting gas, preferably oxygen alone, is used to project the fine oxidizable particles, and preferably also the other particles, towards the substrate to be coated; in other words, the particles are entrained in a stream of such gas which is discharged from a tube or tubes so as to spray the particles onto the substrate. In that case the process is particularly easy to perform and to control.

In the most preferred embodiments of the invention the oxidizable and other particles are of such composition that at least the major part of the resulting refractory mass is composed of at least one oxide. Refractory oxide masses are the most satisfactory for achieving the principal purposes of the invention.

Preferably the fine oxidizable particles are all below 50 microns in size, size meaning the maximum linear dimension of a particle. The best results are obtained with grains of an average size of less than 10 microns.

' The fine oxidizable particles preferably have a specific surface, i.e. surface area to weight ratio, of at least 500 cm /gram. This has been found to be a very advantageous feature for obtaining high quality refractory coatings. This is so particularly, but not exclusively, when forming refractory oxide coatings starting with a mixture of particles of a cumbustible metal or metalloid with particles of one or more refractory oxides, such as those hereinafter specified as particularly satisfactory starting materials.

in the most advantageous embodiments the fine oxidizable particles have a specific surface very much higher than 500 cm /gram. More specifically, particular preference is given to the use of fine oxidizable particles having a specific surface above 3,000 and, optimally, above 9,000 cm /gram.

The size of the other particles can, if they are to undergo melting or softening, be chosen with regard to the degree of melting or softening required under the given temperature and speed conditions. It is in general advantageous for these other particles'to have an average size. of less than 500 microns. Preferably thev said other particles are all below 500 microns in size and, according to preferred embodiments, these particles are wholly or mainly below 300 microns in size.

The advantage of observing the aforesaid preferred upper limit for the size of the other particles is that a rapid melting is promoted. It is often better to use meltable particles which are in a size range up to 500 microns but of which a significant proportion are less than 100 microns, than to use particles which are substantially all about 200 to 300 microns. In some cases it is cheaper to prepare particles to the broader size range specifications.

Advantageously, at least one third by weight of the total quantity of particles of both substances projected against the surface consists of particles above 200 microns in size, and at least one quarter by weight of such total is constituted by particles below 100 microns in size. in many cases it is suitable for the total solid starting material to incorporate 35-40 percent by weight of particlesless than 100 microns in size. By reason of the presence of a significant proportion of very fine particles the complete envelopment of the relatively larger particles by molten substance is promoted.

It is not necessary for the relatively larger particles to be melted otherwise than at their surfaces because melting of the particle cores will not contribute to the cohesion of the mass. Thepresence of the relatively larger particles is therefore an advantage from the point of view of heat economy because these particles absorb less heat per unit of weight.

The process may e.g. be performed so as to form a refractory mass composed of an oxide or oxides present in the starting material, and another oxide or oxides formed by oxidation during the process of the fine particles in the starting material. Alternatively, the starting material may comprise fine particles of an element or elements, togetherwith an oxide or oxides of the same element or elements and the formed refractory mass may in that case be composed entirely of the oxide or oxides used in the starting material.

There may be chemical combination between different oxides in the formation of the refractory mass.

Refractory oxide coatings are particularly suitable for repairing or strengthening furnace linings. The formed coating can be a glass or glasses, which have very high melting temperatures, a refractory ceramic or ceramics, or a composition containing a mixture of vitreous and crystalline phases.

The fine oxidizable particles may be, and preferably are, particles of a metal, metalloid, or a sub-oxide thereof. These classes of substances include the substances hereafter specified which have been found to be particularly suitable combustibles. The purity of the metal or metalloid is preferably at least percent, for example or 97 percent, because this permits the composition of the coating to be made to conform to a predetermined formulation to the virtual exclusion .of unwanted oxides prejudicial to the required refractory properties.

' The following are very satisfactory substances for the fine oxidizable particles: aluminum, magnesium, silicon and zirconium. Those are the most preferred combustibles. Other combustible substances which can be used with good results are: calcium, manganese and iron. Of course a mixture of two or more of such combustible substances can be used.

The other particles are preferably metal oxide particles'or particles of a metalloid oxide or particles of a ternary compound combining two elements in addition to oxygen.

Particularly good results can be achieved using particles of one or more of the following refractory compounds: Al O MgO, SiO ZrO ZrSiO A1 0 SiO 3Al O .2SiO also spinels composed in part of A1 0 and in part MgO or Cr O As an alternative to these substances a more complex substance such as clay can be used.

Particularly beneficial starting materials are those comprising at least one of the metals aluminum and zirconium and at least one compound of oxygen with at least one of such metals, at least 70 percent by'weight of the metal in the solid starting material being present in at least one oxygen-containing compound and at least 10 percent by weight of the metal in the solid starting material being present in the metallic state.

The starting material preferably comprises at least 20 percent by weight, e.g., 20 to 30 percent by weight, of the .fine oxidizable particles. In any event, the starting material preferably comprises at least 70 percent by weight of particles other than the fine oxidizable particles, such other particles preferably being metal oxide particles. The proportion of fine oxidizable particles required depends of course, inter alia, on the amount of heat which must be evolved by the combustion. This in turn depends on various factors such as the initial temperature of the surface against which the particles are projected, the melting, softening or reaction temperature of the other particles, the delivery rate of the fine oxidizable particles and the capacity of the surface and surrounding medium to carry off the heat of combustion. The amount of heat required to be generated by combustion also depends on the surface area against which the particles are projected. For example, when forming a coating, if the jet or spray containing the particles has to be moved to and fro so as to sweep the coating area, he temperature at any given part of the coating zone will intermittently fall and this means that in order to achieve a given result the heat of combustion must be higher than would be necessary if the jet or spray were directed against a single part of the coat unburned oxidizable substance remaining in the coating. Particularly when using a metal as the combustible substance, it is generally advantageous for the whole of this metal to be oxidized because the oxide generally has better refractory properties than the metal. In those circumstances the use of excess metal constitutes a needless expense.

If the jet or spray containing the particles has to be moved to and fro, as mentioned above, the rate of this movement should be such as to avoid flow of the deposited coating material if the orientation of the substrate surface is such as to permit this flow.

For forming refractory coatings it is preferable for the heat evolved by the combustion reaction to raise the temperature of the substrate sufficiently to soften or melt the surface on which the refractory coating is deposited, as well as melting or superficially melting or softening the particles other than those of the combustible substance, and possibly causing different constituents of the projected mass of particles to react together chemically. The melting or softening of the substrate surface. leads to a particularly good adherence of the coating to the substrate. It is also advantageous for the fine oxidizable and other particles to be selected so that the resulting deposited coating is of the same chemical composition, or is of a composition of the same type, as the substrate so that there is continuity in composition, and preferably also in structure, between the substrate and the coating.

The substrate may be pre-heating to a high temperature and/or the particles may be projected against the substrate in a hot atmosphere, to cause ignition, i.e. to start the combustion reaction.

The invention can be used for repairing or strengthening a furnace wall or lining in situ, either at the inner or the outer face of the wall and in the latter case either by starting at the site of a faulty joint which has low heat resistance and has become red hot, or by starting the combustion using a torch to heat the furnace wall or to ignite the oxidizing gas jet.

The advantage of using the invention for repairing industrial furnace walls while the furnace is in operation is considerable becausethe furnace can be kept in operation for anumber of years without stopping production and without any deterioration in the quality of the product produced in the plant. This advantage is particularly important in relation to tank furnaces for glass making. The glass-melting furnace vault or superstructure can be repaired while the furnace is in use and Without polluting the glass in the furnace. The furnace operation'does not have to be disturbed in an way save possibly by stopping a particular burner or particular burners in the immediate vicinity of the spot where the repair is effected, for the brief period of the repair treatment. Wear cavities more than cm in size can be easily filled.

As already mentioned, an additional heat source may be used not only for ignition purposes but also for maintaining the temperature at the coating zone sufficiently high. Such additional heat can be provided by one or more flames directed against the surface of the substrate to be coated and/or at the zone through which the pulverulent starting mixture passes before reaching the substrate. Instead of or in addition to one or more flames, one or more electric arcs can be used. The additional heat source can normally be switched off as soon as the coating operation commences because the heat evolved in the combustion reaction will itself maintain the requisite temperature conditions.

The use of an additional heat source is sometimes desirable, for example, when forming a coating composed entirely of a particular oxide such as MgO such that it would be dangerous to incorporate a sufficient proportion of the pure metal in the starting mixture to provide the entire amount of heat required at the coating zone.

When carrying out the invention by means of oxygen or a gas with a high oxygen content, certain accident hazards are involved and, according to another aspect of the invention, apparatus having a special safety feature is provided.

Apparatus according to the invention basically includes a discharge tube connected with means whereby solid particles and an oxidizing gas can be continuously delivered to such tube to cause discharge of the gas and entrained particles therefrom, and means which is at least indirectly responsive to the advent of an operative condition or conditions indicating a risk of flash-back and'which in the event that such a condition or conditions occur operates automatically to establish safe conditions.

The responsive means may operate to terminate the discharge of a normal combination of starting materials essential for combustion to occur. For example the responsive means may operate to terminate the delivery of oxidizing gas. As an alternative or in addition the responsive means may operate to terminate the supply of combustible particles.

It is preferable to provide means which operates to at least terminate the supply of oxidizing gas and also to flush the system with a gas, e.g., nitrogen, which does not support combustion.

For detecting the advent of unsafe conditions, various types of sensing devices may be used. For example, the responsive means may comprise a device which is at least indirectly responsive to a decrease below a predetermined value in the amount of gas discharging from the discharge tube.

As an alternative, or in addition, the responsive means may comprise a device sensitive to the gas pressure prevailing in the discharge tube upstream of its discharge end. Such a device can yield a control impulse or signal if the oxidizing gas supply pressure falls below a certain predetermined safe minimum value.

The advent of unsafe conditions may also be detected by providing a device sensitive to the temperature prevailing in at least a section of the path along which gas flows before discharging from the discharge tube. For sensing a potentially dangerous rise in temperature in the gas path a fuse may be provided with melts or cuts out if the flame moves back to the location of the fuse, which location may, e.g., be within a pipe section near to the tube discharge orifice.

Of course the responsive means may comprise one or more devices in addition to a device or devices operating as above referred to. For example, the means may comprise a device which reacts to a prohibitive fall in pressure of an available supply of flushing gas, or to a rise in temperature in a cooling circuit for cooling the discharge tube.

It is important for the best results to provide for a well controlled dosage of solid particles into the entraining gas stream. This can be achieved according to the invention by apparatus which includes a hopper for particles contained in the lower layers of the hopper when this filled with pulverulent material.

The particles in the starting material and'the surfaces of the apparatus with which they come into contact should preferably be free from traces of grease, oil or dangerous organic materials which would create a risk of premature ignition, at least when the particles are projected in an entraining stream of oxygen or a gas with a high oxygen content.

An embodiment ofv an apparatus according to the invention will nowbe described with reference to drawing FIG. 1 which shows the apparatus, partly in section. The object to be achieved is to fill a cavity 1 which has been worn in the inner surface ofa wall of the furnace 2 along a joint 3 between wall blocks 4.

The apparatus includes a lance or discharge tube 6 used for spraying refractory material, a powder distributor 7, feed ducts and safe devices.

The lance 6 is formed of three coaxial pipes: a pulverulent mixture entrained by oxygen is sprayed from a central tube 8 which is preferably made of rust resistant steel. Two outer tubes 9 and 10 cooperate with the tube 8 to form a cooling jacket connected to a water-feed 1 l and an outlet 12. The lance 6 can be introduced as shownthrough an aperture l3 in another wall 14 of the furnace 2. The illustrated lance 6 is straight. A curved lance can be used for reaching places which are not opposite a wall opening. For example by means ofa suitably curved lance it is possible to reach a place on the inner surface of the same wall through which the lance is inserted. j

The lance 6 is connected via flexible hose 16, of rubber, for example, anda rigid pipe 18, 19 to the distributor 7 to which oxygen is supplied via a conduit 22 connectedtoa feed 24.. I

The distributor 7 is preferably made of copper or a copper alloy or rust-resistant steel. it is composed of an outer casing 26 having a cover 28 formed with an aperture 30 which can be used for introducing the pulverulent mixture. The casing 26 contains a hopper 31 for holding the pulverulentmixture. Disposed beneath the casing 26 is a motor step-down gearing 32 for producing a low speed rotation of a vertical shaft 34 disposed along the vertical axis of the hopper 31 and extending into the casing 26 via packing 36. The shaft 34 carries a horizontal disc 38 and a frame 40 the shape and size of which are such that it lies adjacent the inner surface of the peripheral wall of the hopper 31. By its rotation the frame prevents the pulverulent mixture from hanging up in the hopper 31. The frame 40 can be formed by a flat section bar the leading edge of which at either side of the frame is tapered to facilitate its movement through the contents of the hopper. The pulverulent mixture continuously discharges from the bottom of the hopper and forms a heap 42 on the disc 38. During rotation of the. disc, a scraper 44 pushes material from the heap at a substantially constant rate into a funnel 46 forming the end of a conduit 19 extending through the wall of the casing 26. The scraper 44 is mounted, preferably with provision for adjustment in all directions, on an arm 48 carried by an attachment 50. The mounting includes two' systems of slots and winged screws 52, 54 which permit the required adjustments. The rate at which pulverulent mixture is displaced from the disc 38 into funnel 46 is adjusted by adjusting the position of scraper 44 and the speed of motor 32. The displaced mixture is entrained by a flow of oxygen passing through the casing 26 and escaping via the path constituted by conduits l9, 18, 16, 8.

The apparatus described is more particularly suitable for continuously entraining a uniform feed of powder, without the risk of clogging. More particularly, the apparatus is suitable for entraining powders which are difficult to entrain, for instance, industrial powder mixtures of unfavorable grain size, such as mixtures comprising very fine grains and larger grains. The apparatus also works satisfactorily in cases in which the mixture has poor flow properties either because of theshape of the grains, or due to humidity, or for some other reason. I

The apparatus embodies a number of safety systems. A graphite bursting disc 56 is mounted on a tube 58 in the cover 28-to prevent the casing 26 from exploding as a result of excess pressure, for instance if the powder gets hot and catches fire in the casing 26.

A fuse 60 having a low melting temperature, for instance 90C, is disposed in the conduit 18 and blows if there is any flashback, i.e. movement of the combustion front of the mixture of oxygen and pulverulent metal upstream within the duct 8,16,18.

A bimetallic electric contact strip 62 is disposed in the cooling water circuit of the lance 6 so as to detect any overheating thereof, e.g., by detecting vaporisation of any water due either to cessation or considerable reduction of the water circulation.

A control box 80 houses electric control circuitry to which the fuse 60, the bimetal strip 62 and the motor 32 are connected and which is connected to a source of E.M.F. via conductor leads8l. The control circuitry is also linked with a manometer unit 64 which is responsive to the gas pressure in a branch conduit 66 for the feed of nitrogen gas, and with valves 68 and 70 in the oxygen and nitrogen feed conduits respectively. The control circuitry is also linked with a control valve 74 in a vent pipe 76 adjacent the lance 6, and with a further manometer 82 which is responsive to the gas pressure in lance 6. The 'control circuitry operates to close the oxygen valve 68 and to open nitrogen valve 70 in the event of the melting or cutting out of fuse 60, the displacement of bimetallic strip 62 or the absence of an operating voltage on the control circuitry. In the event that the oxygen pressure as registered by manometer 82, deviates from a predetermined value, the control circuitry likewise closes the oxygen valve and opens the nitrogen valve 70 and at the same time stops the motor 32. The opening of the nitrogen valve 70 is always accompanied by opening of the valve 74 so that the system is flushed with nitrogen part of which vents to atmosphere and part of which discharges through the lance, assuming that this is not clogged. The flushing of the system with nitrogen prevents any dangerous conflagration. Should a blockage occur upstream of the pipe 76 simultaneously with a flash-back, the nitrogen can escape at the location of the fuse 60 due to the provision at that spot of an insert 78 which is made of the of supervising personnel.

The following are some examples of processes according to the invention.

Example 1 i A mixture was prepared comprising 2022 percent by weight of silicon in the form of particles having a maximum size of 10 microns and a specific surface of 4,000 cm lgram, the remainder of the mixture consisting of sand in the form of particles having a mixmum size of 200 microns. A satisfactory refractory adherent coating of SiO was formed by projecting this particle mixture by means of the above-described apparatus, at a rate of 1 kg per minute in a stream of oxygen delivered at 200 litres per minute onto a silica furnace roof which was at a temperature of l,400C.

Example 2 A mixture was prepared corresponding with that specified in Example 1 except that the sand was of different grain size. In the present example two-thirds of the sand was 200 to 500 microns in size and the rest was 40 to 60 microns in size. A coating was formed by using this mixture in the same way as the mixture specitied in Example 1. The mechanical resistance of the deposit was better than that formed by Example 1. Example3 A mixture was prepared comprising 20-25 percent by weight of aluminum grains with an average size of not more than 10 microns or flakes with a specific surface of 500-10,000 'cm lgram, the remainder of the mixture being alumina particles 200 to 300 microns in size. A worn furnace lining formed of refractory blocks composed of alumina or alumino-silicious material and having atemperature of at least l,OC can be coated by a process and with an apparatus according to the in vention using this mixture as the starting material. The coating formed is a refractory adherent alumina coating. A suitable feed rate is-0.5 kg of the starting mixture per minute in a stream of oxygen delivered at 80 litres per minute. Even better results can be achieved if one half of the alumina is replaced by alumina particles with an'average grain size of 60 microns or less. Example 4 An adherent coating composed of approximately SiO 45% ZrO and 45% A1 0 was formed on electro-cast Zac blocks having a temperature above 1,000C by a process according to the invention, using apparatus as hereinbefore described, and using as the starting material a mixture comprising 17 percent zircon, (ZrSiO 39,0 percent zirconia (ZrO 22.5 percent alumina (A1203), each such ingredient being in the form of particles between 50 and 500 microns in size, 3.5 percent silicon as in Example 1, and I8 percent aluminum in the form of either grains or flakes as in Example 3, and using the same feed rates of the solids and of oxygen as in Example 3.

Example 5 Refractory adherent coatings composed of ZrO and A1 0 were formed on electro-cast Zac blocks by a process and with an apparatus according to the invention using, at the feed rates specified in Example 3, the following starting mixture:

Average grain size in microns Amount by Weight r0 5 26.5 ZrO -250 26.5 A1 0 10 7.5 A1 0 250-300 17.5 Al 5 22.0

Example 6 Refractory coatings composed of MgO.Al O and MgO were formed on basic refractory blocks composed mainly of magnesia preheated to above 1,000C

by a process according to the invention using a starting mixture comprising 20-22 percent aluminum grains 0r flakes as in Example 3 and at the same feed rates, the remainder of the mixture being formed by particles of electrocast or calcined magnesia with a grain size of 200 to 300 microns. Example 7 A coating on refractory blocks composed mainly of magnesia was formed more rapidly than by the procedure according to Example 6 by using a mixture of particles comprising 45 percent electro melted MgO grains 40 to microns in size, 43 percent calcined A1 0 grains having an average size of 20 microns and 12 percent grains of aluminum approximately 15 microns in size and having a specific surface of about 1,000 cm gram. With this mixture the partial volatilisation of MgO was appreciably reduced. The formed deposit was essentially composed of a spine] of eutectic composition comprising 55 percent Al O and 45 percent MgO.

Example 8 Plates were formed according to the invention by projecting a mixture as specified in Example 4, in an enclosure pre-heated to l,l00C, against a vertical surface of a Zac plate disposed above a silica tank into which the composition forming against the said plate flowed down in liquid state into the tank.

The starting mixtures specified in the foregoing examples can be used for forming coatings on laboratory or industrial furnace linings and other structures damaged -by wear, the surfaces to be coated being preheated to or being at at least l,0O0C. Similar coatings can be formed on masonry at higher temperatures, e.g., l,400C.

FIG. 2 illustrates one simple embodiment of the control circuit 80. This arrangement essentially includes four relays 101, 102, 103 and 104 each having its coil connected to a respective sensor. in this embodiment, the sensors 62, 64 and 82 are schematically represented as normally closed switches which open upon the occurrence of the condition for which they are to produce a response. Sensor 64 is connected in series with the coil of relay 101, sensor 62 is connected in series with the coil of relay 102 and sensor 82 is connected in series with the coil of relay 103, the other terminal of each sensor being connected to a suitable voltage source 110. Connected in series between voltage source 110 and the coil of relay 104 is the fuse 60 which will present a short circuit until it is melted. The other end of each of the relay coils is grounded.

A second voltage source 112 is connected via the contacts of the relays to valves 68, 70 and 74, the latter two valves being required to operate together. Valve 68 may be of the normally closed type, i.e. it will be closed when no operating voltage is applied thereto. Similarly, valves 70 and 74 may be of the normally open type, that is they will be open when no operating voltage is alarm by simply monitoring the voltages applied to valves 68, 70 and 74 and motor 32 to produce such alarm if any of those voltages should fall to zero.

face, comprising:

applied to them. The current path for valve 68 is provided via a switch contact of each of relays 101, 103 and 104 while the current path for valves 70 and 74 is established through separate contacts of relays 102, 103 and 104. Finally, the power to motor 32 which is supplied from the source 81 is established through a third set of contacts of relay 103.

The connections between voltage source 112 and Such a circuit could'be provided with a suitable It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

We claim:

1. Apparatus for forming a refractory mass on a sura discharge tube;

a closed chamber having an inlet opening and an outlet opening;

a source of particles of at least one oxidizable substance which burns by combining with oxygen with accompanying evolution of heat and of particles of at least one different substance, at least one of the substances being of a composition such that under the heat of combustion of said oxidizable substance a coherent refractory mass is formed from said two substances, said source being disposed in, and completely enclosed by, said chamber,v and being disposed to deliver such particles to said chamber outlet opening;

a source of an oxidizing gas;

first delivery means connected between said oxidizing gas source and said inlet opening of said chamber for supplying such gas to the interior of said chamber to cause such gas to flow through said chamber outlet opening with such particles entrained;

second delivery means connected between said tube and said chamber-outlet opening for continuously I conveying such solid particles and such gas to said tube to cause discharge of said gas therefrom with said particles entrained and projection thereof against such surface under conditions which cause the oxidizable substance to burn in the presence of the oxidizing gas during such projection;

condition sensing means connected for sensing a condition indicating a risk of flash-back in said second delivery means;

control means connected between said condition sensing means and at least one of said sources for terminating flow therefrom upon the occurrence of 5 s such condition;

a source of gas incapable of supporting combustion;

and

controllable valve means connected between said last-recited source and said second delivery means;

wherein said condition sensing means serve to sense at least one of the conditions of temperature and pressure in said second delivery means, and said control means are connected to said sources of gas for terminating delivery of such oxidizing gas and opening said controllable valve means to deliver the gas which is incapable of supporting combustion to said second delivery means when such condition indicating risk of flash-back is sensed.

2. Apparatus according to claim 1 wherein said control means are further connected to said sources of particles for terminating delivery of such particles when such condition indicating risk of flash-back is sensed.

3. Apparatus according to claim 2 wherein said second delivery means further comprises a normally closed outlet passage near its point of connection to said tube and control means is connected to open said passage when such condition is sensed.

4. Apparatus according to claim 1 wherein said source of particles comprises a hopper containing a mass of such-particles and having an outlet communicating with said chamber outlet opening, and means for controlling the rate at which particles leave said hopper. v

5. Apparatus according to claim 4 wherein said hopper comprises means which is displaceable for imparting relative movement to particles contained in the lower layers of the hopper when the latter is filled with particles.

6 Apparatus according to claim 1 wherein said control means operates to terminate the supply of oxidizing gas.

7. Apparatus according to claim 6 wherein said control means operates also to flush said second delivery means and said tube with a gas which does not support combustion.

' 8. Apparatus according to claim 1 wherein said control means operates to terminate the supply of particles.

9. Apparatus according to claim 1 wherein said sensing means is at least indirectly responsive to a fall below a predetermined value in the amount of gas discharging from said discharge tube.

10. Apparatus according to claim 9 wherein said sensing means comprises a gas pressure sensing device connected to sense the gas pressure prevailing in said discharge tube upstream of its discharge end.

11. Apparatus according to claim 9 wherein said protective means comprises a device connected to be sensitive to the temperature prevailing in at least a section of said second delivery means.

12. Apparatus according to claim 1 1 further comprising a cooling circuit mounted for cooling said discharge tube and wherein said sensing means further comprises a device connected to be sensitive to the temperature prevailing in said cooling circuit. 

1. Apparatus for forming a refractory mass on a surface, comprising: a discharge tube; a closed chamber having an inlet opening and an outlet opening; a source of particles of at least one oxidizable substance which burns by combining with oxygen with accompanying evolution of heat and of particles of at least one different substance, at least one of the substances being of a composition such that under the heat of combustion of said oxidizable substance a coherent refractory mass is formed from said two substances, said source being disposed in, and completely enclosed by, said chamber, and being disposed to deliver such particles to said chamber outlet opening; a source of an oxidizing gas; first delivery means connected between said oxidizing gas source and said inlet opening of said chamber for supplying such gas to the interior of said chamber to cause such gas to flow through said chamber outlet opening with such particles entrained; second delivery means connected between said tube and said chamber outlet opening for continuously conveying such solid particles and such gas to said tube to cause discharge of said gas therefrom with said particles entrained and projection thereof against such surface under conditions which cause the oxidizable substance to burn in the presence of the oxidizing gas during such projection; condition sensing means connected for sensing a condition indicating a risk of flash-back in said second delivery means; control means connected between said condition sensing means and at least one of said sources for terminating flow therefrom upon the occurrence of such condition; a source of gas incapable of supporting combustion; and controllable valve means connected between said last-recited source and said second delivery means; wherein said condition sensing means serve to sense at least one of the conditions of temperature and pressure in said second delivery means, and said control means are connected to said sources of gas for terminating delivery of such oxidizing gas and opening said controllable valve means to deliver the gas which is incapable of supporting combustion to said second delivery means when such conDition indicating risk of flashback is sensed.
 2. Apparatus according to claim 1 wherein said control means are further connected to said sources of particles for terminating delivery of such particles when such condition indicating risk of flash-back is sensed.
 3. Apparatus according to claim 2 wherein said second delivery means further comprises a normally closed outlet passage near its point of connection to said tube and control means is connected to open said passage when such condition is sensed.
 4. Apparatus according to claim 1 wherein said source of particles comprises a hopper containing a mass of such particles and having an outlet communicating with said chamber outlet opening, and means for controlling the rate at which particles leave said hopper.
 5. Apparatus according to claim 4 wherein said hopper comprises means which is displaceable for imparting relative movement to particles contained in the lower layers of the hopper when the latter is filled with particles.
 6. Apparatus according to claim 1 wherein said control means operates to terminate the supply of oxidizing gas.
 7. Apparatus according to claim 6 wherein said control means operates also to flush said second delivery means and said tube with a gas which does not support combustion.
 8. Apparatus according to claim 1 wherein said control means operates to terminate the supply of particles.
 9. Apparatus according to claim 1 wherein said sensing means is at least indirectly responsive to a fall below a predetermined value in the amount of gas discharging from said discharge tube.
 10. Apparatus according to claim 9 wherein said sensing means comprises a gas pressure sensing device connected to sense the gas pressure prevailing in said discharge tube upstream of its discharge end.
 11. Apparatus according to claim 9 wherein said protective means comprises a device connected to be sensitive to the temperature prevailing in at least a section of said second delivery means.
 12. Apparatus according to claim 11 further comprising a cooling circuit mounted for cooling said discharge tube and wherein said sensing means further comprises a device connected to be sensitive to the temperature prevailing in said cooling circuit. 